Galileo Galilei was an Italian polymath: astronomer, physicist, engineer, philosopher, and mathematician. He played a key role in the scientific revolution of the seventeenth century. Galileo built telescopes which had an exceptional and significant impact on our history, the telescopes were used by Galileo to study and discover objects in the sky. Through observation Galileo confirmed Copernicus’s idea of a Sun-centred universe. Galileo also discovered sunspots, craters, and mountains on the moon using the telescope. He also found the stars of the Milky Way, of all his telescope discoveries, he is perhaps most known for his discovery of the four most massive moons of Jupiter now called the Galilean moons. Four Hundred Years Ago, Galileo's